Electrostatic loud-speaker



Feb. 18, 1930. E.'REISZ ELECTROSTATIC LOUD SPEAKER Filed Dec. 26, 1928AT Tm NYs UNITED s EUGEN BEISZ, BERLIN-DAHLEM, GERMANY ELECTROSTATICLOUD-SPEAKER Application filed December 26, 1928, Serial No. 328,598,and in Germany December 29, 1927.

' This invention relates to electrostatic loud speakers and the like,and more particularly to electrostatic loud speakers of the kind inwhich a soft ductile membrane, provided duced by the loud speaker are nolonger proportional to the applied electrical amplitudes and distortionconsequently occurs.

According to this invention the rigid conductin late of an electrostaticloudspeaker of the 1nd referred to is formed with holes or perforationsof such shape that, even with large amplitude vibrations, theelectrostatic forces upon the membrane, and therefore the movementthereof, remain proportional to the applied electrical amplitudes.

The invention is illustrated in the accompanying drawings in which VFigure 1 shows the essential portions of a loud speaker in accordancewith the sald 1nvention, I

Figures 2 and 3 are enlarged fragmentary views of a portion of the saidloud speaker with its membrane in two different positions, while Figure4 indicates diagrammatically the chief positions of the membrane durlngits oscillations,

Referring to the drawings, a is a membrane, for example, of rubber, uponwhich is afiixed a thin layer 6 of a conducting ma terial, as, forexample, carbon particles or a very thin metal foil. The membrane a isplaced against a rigid metal plate 0 provided with holes 6 whoseexterior walls f are curved parabolically as shown. The exact form ofthis curve, i. e.- the parabola, determines the distribution of theelectrostatic field, and. the

/ into the positions shown at w and If the dimensions of the said curveWill clearly de pend upon the stiflness of the membrane a (taking intoaccount, of course, the coating 6) and the value of the constantcomponent of the electrostatic field, i. e. the value 5; of the biasfield. This bias field, which isdue to the constant component ofpotential applied to the loud speaker, and the value of the rigidity ofthe portions a and I), determine the exact course of the line f. 6( Ithas been found, as a result of a large number of experiments, that forthe desired result the curve f should be a parabola, in which stiffnessof the membrane and electrostatic bias of the loud speaker are thevarie5 ables. Obviously,vthe dielectric constant of the insulatingmaterial employed, i. e. of the membrane, will afiect the dimensions ofthe parabola, but since the experiments referred to have all been madewith one and the same 7( insulating material as membrane, thisconsideration is not contained in the parabola equation.

It will be seen that when the membrane oscillates towards and away fromthe open- 72 ings e in the solid metal electrode 0 of the loud speaker,a large number of small separate membranes is, in effect, produced, thesaid small membranes being in the untensioned condition bounded by theline w 8( (Figure 4). If, now, tension is applied, the flexible member awith its conducting coating 6 approaches the metal electrode and takesup the position shown by the curve y. Under the effect of varyingapplied potentials (acoustic potentials) between the electrode 0 and thecoating 12, the limiting curve moves to and fro, that is to say, themembrane moves exterior Walls of the openings 6 are of parabolic form,as indicated in Figure 4, then the electrostatic field between theelectrode and coating 1) will remain the same during movements of themembrane, since the air spaces 9 between the said plate a andoscillating membrane remain the same. In this way the loud speakeroperates satisfactorily even when the magnitude of the potentialamplitudes is very large. 10

I claim:

1. In anelectrostatic acoustic device, a metallic plate having openingsthrough the same, a soft ductile membrane, and a coating 5 of aconducting material on one side of said membrane, said membrane beingarranged I with its uncoated side opposed to an adjacent side of saidplate to produce a structure hav ing the characteristics of a condenser,the Walls of the said openings being shaped to substantially parabolicform toward that side of said plate opposed to the said membrane.

2. In an electrostatic loud speaker or the like, a rigidmetallic platehaving openings through the same, a soft ductile membrane and arelatively thin coating of a conducting material on one side of saidmembrane, said membrane beingarranged with its uncoated side directlyopposed tovan adjacent side of said plate to produce a'structure havingthe characteristics of a condenser, the Walls of the said openingstoward the ends thereof at the side of said plate opposed by saidmembrane being shaped to substantially parabolic form. In testimonywhereof I have signed my name to this specification.

EUGEN REISZ.

